Method and apparatus for controlling the cooling zone of a tunnel kiln



Aug. 4, 1964 o. H. cRoss 3,142,884

METHOD AND APPARATUS FOR CONTROLLING THE COOLING ZONE OF A TUNNEL KILN Filed Oct. 19, 1961 4 Sheets-Sheet 1 D II LI INVENTOR OWEN H. CROSS ATTORNEYS.

Aug. 4, 1964 o. H, cRoss METHOD AND APPARATUS FOR CONTROLLING THE COOLING ZONE OF A TUNNEL KILN 4 Sheets-Sheet 2 Filed Oct. 19, 1961 mm M .2 Wm m d w u" 9 u lllll w l l I I I I 1 l l I ll all, mm mm l l l llllullil Ill-Ill :P P L w T L 3 E TELL": I IlLIILt IL iiff; I: ;I T I I n T. w I I v 1.. :1 u I 1 m" IHITIEHLI L. i I l I I llll u on \|\l.\ mm x N N m w F mv mv mm mm on mm ATTORNEYS.

Aug. 4, 1964 o. H. cRoss 3,142,884

METHOD AND APPARATUS FOR CONTROLLING THE COOLING ZONE OF A TUNNEL KILN Filed on. 19, 1961 4 Sheets-Sheet a FIG. 4.

INVENTOR OWEN H. CROSS ATTORNEYS.

Filed Oct. 19, 1961 4, 1964 o. H. cRoss 3,142,884

METHOD AND APPA us FOR CONTROLLING THE COOLING 20 OF A TUNNEL KILN 4 Sheets-Sheet 4 FIG. 5.

INVENI'OR OWEN H. CROSS ATTORNEYS.

United States Patent 3,142,884 METHUD AND AIPARATUS FGR CQNTRGLLIWG THE CGGLWG ZONE OF A TUNNEL KILN Owen H. tlross, QHara Township, Allegheny County,

Pa, assignor, by mesne assignments, to Pullman Incorporated, Pittsburgh, Pa., a corporation of Delaware Filed (let. 1?, 1961, Ser. No. 146,159 7 Ciairns. (Cl. 2S142) This invention relates to tunnel kilns, and particularly to cooling zones of tunnel kilns. More particularly, this invention relates to the portion of the cooling zone immediately adjacent the firing zone and methods and apparatus for controlling the temperature of such zone.

Tunnel kilns are elongated bodies of refractory material for the passage of ceramic ware therethrough. Generally, a tunnel kiln has three comparative well defined zones, a preheat zone, a firing zone and a cooling zone. In the preheat zone the ware is gradually heated to a temperature close to the firing temperature whereby to prevent cracking of the ware which would result in rapid heating thereof. In the firing zone the ware is subjected to very high temperatures for protracted periods of time, and, in many cases, the ware is glazed in the firing zone. Upon leaving the firing zone the ware is cooled in the cooling zone.

The rate of cooling of the ware in the cooling zone is very critical in order to prevent cracking thereof. This is due not only to conventional thermal expansion and contraction, but due to the fact that as the ware cools down it passes through the well known quartz beta-alpha inversion point (1065 P.) where a large volume change takes place. It has been found that if the ware is cooled rapidly through such a point, cracking most certainly will result. In order to insure that the ware is taken through the beta-alpha inversion point at a relatively slow cooling rate, it has been normal procedure hereinbefore generally speaking to very slowly cool the ware through the cooling zone to insure the gradual passage of the ware through the inversion point.

The problems of the crackage of ware as it is heated, fired and cooled in a tunnel kiln is even more critical in the field of sewer pipes, wherein the clay bodies run to huge sizes from several hundred pounds up to several thousand pounds. In bodies of this size local stresses are more apt to occur to thereby cause cracking of the ware. Accordingly, this invention is particularly useful in connection with the manufacture of sewer pipes.

However, it has been known that the ware can be cooled much more rapidly down to a temperature close to the inversion point temperature of 1065 F. and, after the ware has slowly passed through said inversion point temperature, the ware can once again be cooled fairly rapidly. More particularly, it has been found that aside from the possible cracking due to the phase change at the betaalpha inversion point, the thermal stresses set up by relatively rapid cooling elsewhere in the cooling zone will not be sufficient to cause substantial breakage.

The problem of rapid cooling of ware down to a predetermined temperature above the beta-alpha inversion point would not be especially complicated were it not for the fact that the amount of ware traveling through the kiln is not a constant but varies over narrow or wide limits depending upon the particular installation. Hence, it will be necessary to vary the amount of heat removed from the ware in order to be sure that the ware comes down to substantially the same temperature above but close to the inversion point and not below nor greatly above that temperature.

Heretofore, to determine the temperature conditions within a tunnel kiln, it has been known to employ thermocouples, pyrometers or other temperature measuring means which means was adapted to measure the temperature of the gasses in the tunnel kiln. However, the temperature of the gasses within the tunnel kiln are not always dependent upon the temperature of the Ware that is the critical thing which must be observed. For instance, if hot gasses are being poured into a particular zone of a tunnel kiln, the temperature measuring means may measure the temperature of the gas being introduced into the kiln which gas may be substantially hotter than the temperature of the ware itself. This would tend to give an erroneous impression to the kiln operator.

It is therefore one object of the present invention to provide a new and improved tunnel kiln having a cooling zone with apparatus for rapidly cooling the ware exiting from the firing zone to a temperature slightly above the beta-alpha inversion point.

Another object of the present invention is the provision of apparatus for automatically controlling the rapid cooling of ware exiting from the firing zone of a tunnel kiln down to a temperature close to but above the quartz betaalpha inversion point.

Still another object of the present invention is the provision of automatic means for accomplishing the above stated objects and for further controlling the cooling of the ware to insure that it passes through the beta-alpha inversion point at a very slow cooling rate.

Yet a further object of the present invention is the provision of an automatically controlled kiln adapted to cool ware rapidly and evenly in those zones where such Ware can be so cooled and to slowly cool the ware under automatic control in the zone where it passes through the beta-alpha inversion point.

Another and further object of the present invention is the provision of automatic means for controlling the rate of cooling of ware in certain portions of the cooling zone of a tunnel kiln which means includes a radiation pyrometer which measures a temperature that is either directly dependent on or is the temperature of the surface of the Ware passing through the kiln.

The above and other objects, characteristics and features of the present invention will be more fully understood from the following description taken in connection with the accompanying illustrative drawing.

In the drawing:

FIG. 1 is a plan view of a tunnel kiln;

FIG. 2 is a diagrammatic view of the cooling zone of a tunnel kiln including the apparatus for controlling the cooling of ware therein;

FIG. 3 is a longitudinal sectional view of the rapid cooling zone in said tunnel kiln;

FIG. 4 is a sectional view taken along the line 44 in FIG. 3, and

FIG. 5 is a sectional view taken along the line 55 in FIG. 2.

Referring now to the drawings in detail, a tunnel kiln 10 is diagrammatically shown in FIG. 1 and has a preheat zone 12, a firing zone 14, a cooling Zone 16. Ware is passed through the tunnel kiln 10 on kiln cars which run on track rails 18. In order to fire the ware properly, the temperature in the firing zone 14 generally is between about 1900 F. and 2100 F. Accordingly, it is necessary to gradually preheat the ware in the preheat zone 12 to a condition so that it may be soaked at such temperatures while the ware passes through the firing zone 14 and it is necessary to cool the ware in the cooling zone 16 from such firing temperature back close to room temperature.

The cooling zone 16 is shown in diagrammatic detail in FIG. 2.

To facilitate the analysis of the operation of the cooling zone, the cooling zone has been divided into five subzones A, A, B, C and D, which sub-zones may be termed the rapid cooling sub-zone A, which is immediately adjacent the firing zone, first, second and third slow cooling sub-zones A, B and C and the exit sub-zone D.

Referring first to the rapid cooling sub-zone A, the brickwork of the kiln in this sub-zone is shown in FIGS. 3 and 4. The zone A includes a base 17 on which are mounted the track rails 18. Extending upwardly from the base 17 are side walls 2 and 22 which are connected at the top by a roof 23 having two spaced apart roof portions 24 and 26 whereby to define a passage 28 therein. A pair of passages 30 and 32 lead through the roof layer 26 into the space 28. Extending through the roof into communication with the upper portion of the kiln chamber 34 are a pair of passages 36, and 38 defined by brickwork 40 and 41, respectively. Extending downwardiy through the side walls and 22 are fiues 36 which are in communication adjacent the top thereof with the passage 28 and have openings or ports adjacent the bottom.

Disposedwithin the rapid cooling sub-zone A of the cooling zone 16 of kiln 10 is a means for measuring the temperature of the surface of the ware passing through the kiln. As shown herein the ware surface temperature measuring means are radiation pyrometers 46 and 48. However, other means for measuring the surface temperature of the ware may be employed in connection with the present invention. As shown in the drawings, the radiation pyrometers 46 and 48 are disposed in tubes or passages 42 and 44, respectively, disposed in the roof and side walls of the rapid cooling sub-zone A. As will be understood more clearly hereinafter, the radiation pyrometers are designed to control the cooling rate in the rapid cooling zone. Specifically, each side wall 20 and 22 is provided with three vertically spaced tubes 44 which are adapted to mount radiation pyrometers. The roof or crown is provided with one such tube 42. In practice, only one radiation pyrometer is mounted in each wall and one radiation pyrometer is mounted in the roof tube. The other wall tubes are not employed. However, the provision of a multiplicity of wall tubes gives the user the opportunity of disposing his wall radiation pyrometers at levels that he deems best for giving the most accurate reading of temperatures of the ware within the rapid cooling zone. It will also be noted that as is shown in FIG. 3 and as is presently preferred, the radiation pyrometers are not adapted to sight directly on the ware passing through the chamber 34 in the rapid cooling zone A but, instead, they sight on a disc of highly refractory material such as, for instance, a stainless steel disc 50 which is mounted in any suitable fashion as close to the ware as possible as by rods'52. The purpose of the stainless steel'disc 50 is to serve as a means of smoothing out rapid temperature fluctuations which may occur in localized areas ofthe ware. However, the stainless steel disc 50 will represent the average temperature of the ware passing in the vicinity thereof whereby when this temperature is read by the radiation pyrometer an accurate relatively steady reading can be achieved. Of course, if desired, the stainless steel disc or target 50 can be dispensed with,v in which event the radiation pyrometers will sight directly on the ware. However, when such is done, and as will be understood more clearly hereinafter, there may be a tendency for the control apparatus to be described below to hunt or constantly change its conditions, which would be somewhat undesirable. However, this could be avoided by decreasing the sensitivity of the controllers which will be described hereinafter. i

Referring now to FIG. 2 in detail, the rapid cooling zone A is adapted to have cooling air recirculated therethrough, which coolingair is circulated by a fan 54. The

inlet 56 of the fan 54 connects with a duct 58 and with' a cold air inlet 60 which is'controlled by an automatically operable damper 62. The duct 58 communicates with ducts 64 and 66 which extend downwardly into communi- 4 cation with openings 30 and 32 in the roof portion 26 to communicate with the flues 36 on both sides of the chamber. The ducts 64 and 66 have disposed therewithin manually operable dampers 68 and 70, respectively, which dampers are adapted to control the flow through the ducts.

The outlet of the fan 54 is connected to a duct 72 which in turn communicates with a pair of ducts 74 and 76 which are respectively connected to the passages 36 and 38 through the brickwork 40 and 41, respectively. Referring particularly to FIG. 3, the passages 36 and 38 pass down in a vertical manner through the brickwork 40 and 41 and are thence in communication with horizontally directed nozzles 78 and 80, respectively, which nozzles are directed against one another to thereby create some static pressure in the upper portion of the kiln chamber 34. To control the flow of air through the ducts 74 and 76, a pair of dampers 82 and 84, respectively, are provided.

Also connected to the outlet of the fan 54 is a duct 86 which extends to an exhaust duct 88 adapted to carry gasses away from the kiln. For instance, the duct 88 can be vented to atmosphere whereby to release waste heat, or it may go to the preheat zone of the kiln whereby to increase the overall efficiency of the kiln or it may be directed to some other apparatus in this vicinity requiring heat. The specific use to which the hot gasses in the duct 88 are put forms no part of the present invention. However, in accordance with the present invention, a second automatically controlled damper 90 operates to control the flow of hot air through the duct 86.

The circulation of the air in the rapid cooling sub-zone A is as follows: from the exhaust of the fan 54 the air will travel partially through the duct 72 and partially through the duct 86, the relative proportions to be determined by the settings of the manually operated dampers 82 and 84 and the automatically operated damper 90. That portion of air passing through the duct 86 will flow into the exhaust duct 88 and thence out through said duct. The portion of the air in the duct 72 will split and flow through the ducts 74 and 76 and thence down through thepassages 36 and 38 and out through the nozzles 78 and into the top of the kiln chamber 34. As will be understood more thoroughly hereinafter, this air will be relatively cool air. The air will thence pass downwardly through the kiln chamber in close relation with the ware passing therethrough and thence in through the ports or inlets 25 into the tines 36. The air will then pass upwardly through the fines 36 and into the passage 28 in the roof and will be drawn out through the openings 30 and 32 in the top of the roof into the ducts 6.4 and 66. Air will then pass into the duct 58 and thence to the duct 56 and back to the inlet in the fan.

The purposeof the circulation of the air as just described is to cool the ware as it passes out of the firing zone of the kiln so that it will rapidly be brought down from its firing temperature of between about 1900" F. and 2100 F. to a temperature slightly above the quartz beta-alpha inversion point of 1065 F. Preferably the temperature at the exitend of the rapid cooling sub-zone A should be approximately 1100 F. to 1150 F. In order to accomplish this rapid cooling in a short space of time, cold air is bled into the inlet for the fan 54 through the inlet 60 which, as described hereinbefore, is controlled by the automatically controlled damper 62. Thus the air passing through the duct 56 will be a mix-i ture of the hot air which has passed through the kiln chamber and has picked up heat from the ware therein and cold air passing through the inlet 60. It is this relatively cool air mixture which is injected into the top of the kiln through the nozzles 78 and 80. Thusit will be seen that there is a constant inflow of cool air which air moves downwardly through the kiln'chamber 34 of the rapid cooling sub-zone A to pick up heat from the ware and thence out through the bottom of the kiln chamber where it is once again diluted with cool air and recirculated.

It will be noted that the volume of air is constantly being increased by the inflow of additional cold air through the inlet 60. Thus, unless some provision is made for removing air from the recirculating system, there will tend to be a pressure build up in the rapid cooling sub-zone A which pressure build up will tend to cause gasses to flow either toward the exit end and/ or towards the entrance end away from the rapid cooling zone. While this may be desired at all times, the amount of such flow or the presence and absence of such flow can be controlled by controlling the amount of air permitted to flow out through the exhaust duct 86 into the exhaust duct 88. This control of the air flowing through the duct 86 is effected by the automatically controlled damper 90. Further, the ratio of the amount of air being bled into the system through the inlet 60 and the amount of air being bled out of the system through the duct 86 can be maintained as a constant and this is one of the desirable features of the present invention. In accordance with the present invention, a motor means 92 is provided for controlling the damper 62. This motor means is connected to a second motor means 94 which operates to open and close the damper 90. Interposed within the connection between the motor means 92 and 94 is means for insuring that the motor means 94 follows the operation of the motor means 92. This means can be any suitable electrical or electronic means and, in fact, the system including the motor means 92 and 94 and the follower means 96 may be a simple relay or servo system. It will be understood that the ratio of the volumes of air bled in through inlet 60 and bled out through duct 86 can be adjusted manually by fixing the relative positions of the dampers 62 and 90. Once fixed the automatic apparatus will maintain the ratio by effecting concomitant movement of the dampers 62 and 90.

The operation of the dampers 62 and 90 is automatically controlled by the operation of one of the radiation pyrometers 46 or 48, or, as indicated hereinabove, other means for measuring the surface temperature of the ware. Which pyrometer controls is a matter of individual choice for the operator and, as indicated hereinbefore, there are three radiation pyrometers available, two pyrometers 48, one each in each of the side walls, and a pyrometer in the roof which pyrometer is designated by the reference character 46. The manner of selecting which pyrometer will control is by means of a simple three position switch 98. The two pyrometers which are not employed to control are used as checking devices with respect to the controlling pyrometer. The input from the controlling pyrometers 46 and 48 is supplied to a standard controller 109 which is preferably although not necessarily of the recording type available from a number of companies such as Minneapolis-Honeywell Company and Leeds and Northrop. The specific construction of the controller 100 forms no part of the present invention. Suffice it to say when the temperature sighted by the radiation pyrometer rises, the output voltage of the pyrometer will rise to actuate the controller to put out a signal of a given polarity over wire 104. When the temperature sighted drops, the output voltage of the pyrometer drops and at some given reduced voltage will actuate the controller to put out a signal of opposite polarity. The construction of the controller, as stated, forms no part of the present invention and the controller can operate on any one of a number of principles well known in the art. For example, the input from the pyrometer can be compared with an internal reference signal and the difference, amplified, can be supplied to the wire 104. Another example of a mode of operation of a satisfactory controller is to supply the input from the pyrometer to a galvanometer mechanism which operates to open and close contacts that control the supply of energy to the controller output wire 104. Other types of controllers are well known to those skilled in the art. Regardless of the form of controller, the motor means 92 is energized over wire 104 and is responsive to the output signal from controller 100, whereby to open and close the damper 62 in the duct 60 to thereby increase or decrease the amount of cold air bled into the inlet of the fan 54. As the motor means 92 operates in response to the signal put out by the controller 1% it in turn will be followed in its operation by the motor means 94 through the relay or servo system including device 96. As noted hereinbefore, the following of the motor means 94 will operate to open and close the damper whereby to maintain the proper ratio of cold air bled into the system and hot air bled out of the system so as to prevent any unwanted pressure build up or creation of partial vacuum.

As noted hereinbefore, it is desired to have the ware leave the rapid cooling sub-zone A at a temperature of between about 1100 F. and 1150 F. It will be known, therefore, that at the point where it passes within the line of sight of the controlling radiation pyrometer it will have to be at a certain temperature. If the temperature measured by the controlling radiation pyrometer is greater than that determined to be necessary in order to have the ware leave the rapid cooling zone at the desired temperature, then the signal generated by the controlling radiation pyrometer will be such as to cause an opening of the damper 62 whereby to bleed in additional cold air into the fan inlet so as to reduce the temperature of the air being introduced into the top of the kiln chamber by the nozzles '78 and 80. Simultaneously, the motor means 94 will follow the motor means 92 in order to open wider the damper 90 to cause an increase in the bleed out of the system so as to maintain the proper pressure within the rapid cooling zone. f subsequently, ware enters within the line of sight of the controlling radiation pyrometer at a substantially lower temperature than the certain or predetermined temperature, then it will effect a reduction in the amount of cooling by recirculation in the rapid cooling sub-zone which reduction is effected by actuating the controller to operate the motor means 92 in such a fashion as to close the damper 62 whereby to reduce the amount of cold air being bled into the recirculating system. Of course, in order to maintain the proper pressure within the rapid cooling sub-zone A, the damper 90 will also close so as to reduce the amount of air mixture being bled off from the outlet side of the fan into the duct 88 in order to maintain desired pressure within rapid cooling sub-zone A.

It will be understood that the major reason for temperature variation of the ware will not be due to the fact that there is sharp variation of temperature in the firing zone but, instead, due to the fact that the volume of ware passing through is not a constant and, accordingly, the amount of heat to be removed in the rapid cooling zone in order to reduce the temperature of the ware passing through to between about 1100 F. and 1150 F. is a variable. Accordingly, if the amount of cold air bleed is too large for the volume of ware passing through then, obviously, the temperature of the ware will drop too sharply and the ware may well pass through the beta-alpha inverslon point in the rapid cooling zone which would 1mdoubtedly cause almost total cracking. This is to be avoided and the manner of avoiding it is by adjusting the cooling to retard it during the portion of the travel of the ware through the rapid cooling zone beyond the radiation pyrometers so that the ware will leave the rapid cooling zone at the proper temperature. Moreover, it is equally undesirable to have the ware pass out of the rapid cooling zone at too high a temperature which will necessitate too rapid cooling in the critical zone controlled by other apparatus to be described hereinafter which also may Well result in cracking.

It should also be kept in mind that the temperature of the air being fed into the top of the kiln chamber by the nozzle 73 and 80 cannot be too low as if it is there will be local thermal stresses set up in the ware which thermal stresses may cause cracking. It has been found that the minimum temperature of the cool air being fed into the top of the chamber should not be below about 650 F. It will further be noted that by bleeding cold air into the system on the inlet side of the fan, that the fan is never required to propel the air at its maximum temperture, whereby to enable the fan to be: made of materials that are readily available refractories which are not unduly expensive.

In addition to the novel automatically controlled recirculation apparatus described with respect to the rapid cooling sub-zone A additional apparatus is provided, some of which is automatically controlled and some of which is manually controlled, in order to insure that the remainder of the cooling is carried out with minimum cracking of the ware.

As can best be seen from FIG. 5, the portion of the cooling zone outside of the rapid cooling sub-zone A, that is the sub-zones A, B, C and D are all similar in cross section. In the walls 20 and 22, however, there are a plurality of vertically extending flues 106 which are in communication with a roof space 188 that communicates with a chamber 110 in a crown 112 in the top of the roof. The flues 196 are in communication with the outside through passages 114 which may be opened or closed by sliding bricks 116. Beneath the vertical flues 106 are horizontal flues which, depending on their location, are designated by the reference numerals 118, 120, 122. These horizontal flues have a plurality of ports through the walls 20 and 22 leading into the kiln chamber, which ports are designated by the reference numeral 124. In the vicinity of the exit ends of the B and C sub-zones and approximately midway through the subzone D are provided vertical flues which do not terminate just above the horizontal flues 118, 120 and 122 but instead communicate with said flues. These vertical flues are designated in FIG. 2 by the reference characters 126, 123 and 130, respectively. The purpose of the apparatus associated with the zones B, C and D is to introduce cool air at the exit end of the kiln, to warm the air by removing heat from the ware and to advance the warmed air forward one sub-zone where it is removed from and returned to the kiln and then advanced still another zone. In addition, the apparatus maintains the proper pressure conditions within the cooling zone 16 and insures that the ware passes through the beta-alpha inversion point at a very slow cooling rate.

The apparatus associated with the sub-zone A is primarily included to maintain the proper circulation conditions in the cooling zone 16. That is, the apparatus to be described hereinafter mainly performs the function of creating a draft to have air moving from the exit end back towards the rapid cooling sub-zone of the cooling zone 16. As seen best in FIG. 2, a duct 132 extending transversely of the kiln communicates with the chamber 108 in the roof of the kiln in the sub-zone A. The duct 132 after it clears the side wall of the kiln is connected to a fan 134 which has its outlet 136 connected to the duct 88. Thus, with fan 134 operating it will draw air out of the kiln chamber 34 in the sub-zone A, up through a plurality of longitudinally spaced passages 109 into the space 108 and thence to the duct 132 and the fan 134 and the duct 136 to the duct 88. Preferably, the bulk of the volume of the air which is removed by the fan 134 is supplied to the sub-zone A by the horizontal flues 118 which receive their air from the vertical flues 126 communicating therewith which in turn are connected to an external duct 138 connected to the outlet of a fan 140. The fan 140 has its inlet connected to a duct 142 which has disposed therein a manually operable damper 144. The duct 142 is in turn connected to a transversely extending duct 146 which; by means of the roof chamber 110 and a plurality of passages 109 in the sub-zone B, is in communication with the top of the kiln chamber 34 in sub-zone B. Thus the fan 140 will draw air from the sub-zone B through 8 the top of the'roof and supply it to the bottom of the sub-zone A through the horizontal ducts 118. I

The volume air withdrawn from the sub-zone B by the fan 149 is preferably mainly supplied to said sub-zone B by the horizontal flues 120. These horizontal flues 120, as previously noted, communicate with vertical flues 128. The upper ends of the vertical flues 128 are connected to a transversely extending duct 148 which in turn is connected to the outlet of a fan 150. The inlet of the fan 150 is connected to a duct 152 having a manually operable damper 1S4 disposed therein. The duct 152'is in turn connected to a transversely extending duct 156 which is connected to the top of the kiln chamber 34 through the roof passages 109 in the sub-zone C. Thus the fan 150 obtains the air that it supplies to the bottom of the sub-zone B in part at least, from the top of kiln chamber 34 in subzone C whereby to advance air from the sub-zone C toward the sub-zone B.

The air removed from the sub-zone C by the fan 150 through the duct 156 and other parts connected there with is preferably supplied at least in part through the ports in the horizontal flues 122. The horizontal flues 122 are, as previously described, connected to the vertical flues 130. The vertical flues 130 are in turn connected at their tops to external ducts 158 which extend off at angles from a central duct- 160. The central duct 160 is also provided with a number of openings 164 which extend through the roof of the kiln, to supply cool or tempered air being fed from the fan 162 into the top of the exit end D of the kiln. The inlet of the fan 162 is supplied by a duct 166 having two branches, a branch 168 which is vented to the atmosphere and controlled by a manually operable damper 170 and a duct 172 having two longitudinally extending branches 174 and 176 extending along the sides of the kiln at the top. The branches 174 and 176 have ports 178 and 180 which communicate with vertically extending flues 181 having inlet ports adjacent the bottom of the exit end D. Thus the fan 162 serves to draw in cold air from outside and to supply the tempered air resulting from a mixture of the cold air drawn in through inlet 170 and the warm air recirculated through duct 172, to the top of the exit end D of the kiln and to the bottom of the sub-zone C of the kiln and to remove some of the air supplied to the exit end through the ports 178 and 180 whereby to cause some local recirculation of tempered air at the exit end of the kiln to insure a final reduction in the temperature of the ware at said exit end.

As noted hereinbefore, the walls 20 and 22 are provided in the sub-zones A, B and C with a plurality of vertically extending flues 106 which flues are in communication with the space 108 between the roof portions 24 and 26. These vertical flues 106 are all provided with openings or passages 114 to the outside through the side walls. The passages 114 are openable and closable by sliding bricks 116. The purpose of the in: clusion of these elements is to provide a very gradual cooling action which results by virtue of the fact that when any of the fans 134, 140 and 150 are operating they will tend to draw air not only from the kiln chamber 34 through the passages 169 but also from the flues 106, which flues will be supplied with cool outside air through any non-closed openings or passages 114. This passing of cool air up through the flues 106 will serve'to mildly cool the kiln chamber 34 and to thereby supplement the cooling effect resulting from the passage of air directly over the ware. As will be understood more clearly hereinafter, such a mild cooling action may be relied on as the only cooling action should the cooling rate in the sub-zones A B and C be too great due to the circulation of air through the kiln chamber 34'itself.

From the foregoing it will be seen that as the air supplied to the exit end of the kiln is advanced by the apparatus heretofore described, it is generally being circulated in each of the sub-zones in contact with the ware whereby to pick up heat therefrom to cool the ware and to heat the air. Thus as the air is moved further down away from the exit end of the kiln it will become increasingly hot. However, in order to insure that the air is sufliciently hot particularly in the sub-zones B and C burners 182 and 184 are provided which burners discharge their flames into the ducts 142 and 152, respectively, whereby to heat the air being withdrawn respectively from the exit ends of the B and C sub-zones and being discharged into said sub-zones forward of the exit ends. The purpose of the burners 182 and 184 is to boost up the temperature of the circulating air in the B zone and C zone whereby to retard the rate of cooling in the sub-zone A if such retardation is necessary in order to insure that the ware passes through the beta-alpha inversion temperature at a very slow rate. However, if the cooling rate is not retarded sufficiently by the introduction of additional heat through the burners 182 and 184 additional retardation can be eflected by reducing the amount of circulating air as by closing the dampers controlling such circulation or, in lieu thereof, the circulation can be completely stopped and the slow cooling due to the air passing through the flues 106 can be relied on to effect a reduction of temperature in the area of the beta-alpha inversion point.

In accordance with the present invention the control of at least one of the burners 182 and 184 is effected automatically in response to readings of radiation pyrometers 186 in the side walls or the radiation pyrometer 188 in the crown of the C sub-zone of the cooling zone 16. As was true with respect to the radiation pyrometers in the rapid cooling zone, only one of the three radiation pyrometers provided will actually eifect control, the other two being employed for checking on the operation of the one being used. In addition, in the side wall spare passages which are vertically spaced from the employed passage will be provided in order for the user to find the proper location for the pyrometers in the side walls. The device for selecting which pyrometer will act as the controlling pyrometer is a simple three position switch 190, the output lead of which is connected to a controller 192 substantially identical to the controller 100. As was stated with respect to controller 100 the controller 192 is a conventional controller and may be purchased from a number of sources of supply such as, for instance, the Minneapolis-Honeywell Company or Leeds and Northrop. As is conventional with pyrometer controllers, controller 192 will put out a signal over its output lead 196 to a motor means which may be a solenoid or an electric motor herein designated by the reference numeral 198 to effect the turning on and off of the burner 184. If the controlling radiation pyrometer 186 or 188 senses that the temperature of the ware is too high, it will cause the controller to energize motor means 198 to sharply damp or turn off the burner 184. If the temperature on the other hand is too high then the burner 184 will be put on by the motor means 198. In this way the temperature in the precise temperature vicinity of the beta-alpha inversion temperature can be closely controlled in order to be sure that as the ware passes through said inversion temperature it is being cooled at a very slow rate. This will eliminate the tendency of the ware to crack.

As shown herein, the burner 182 is manually controlled and will be turned on and off by the operator in accordance with temperature readings taken by thermocouples, radiation pyrometers or other temperature measuring devices. However, if desired, burner 182 can be automatically controlled in the same manner as burner 184 and, if further desired, by the same apparatus. In the alternative, if desired, the burner 182 can be automatically controlled by apparatus substantially identical to the control apparatus 190, 192 and 198 and the burner 184 can be manually controlled.

It will be understood that once the ware has passed through the critical temperature of 1065 F., which preferably takes place in sub-zone A or sub-zone B, the cooling rate can again be accelerated and such accelerated cooling rate is achieved in the sub-zones B and/ or C and the exit sub-zone D depending on conditions. This will insure that when the ware leaves the kiln it will be at a sufliciently low temperature that there will not be undue thermal stress resulting from its exposure to the atmosphere.

Througout the specification, mention has been made of a number of manually operable dampers. It will be understood that the purpose of these dampers is to control the flow of air and combustion gasses or, if desired, to discontinue such flow altogether. For instance, if it is found that the cooling rate in the B and C zones is so great that it cannot be retarded to proper limits by the full ignition of the burners 182 and 184 and by the complete closing of the passages 114 into the fiues 106, manually operated dampers can be employed to discontinue the supplying of cold air to the horizontal flues 122 from the exit end or, such supply can be sharply reduced by manual operation of the dampers. However, I have not chosen to control automatically such dampers as the variations in temperature demanding the operation thereof are so slow that the difiiculty and expense of automatic control is not deemed feasible. Notwithstanding this, if desired, such manually controlled dampers can be controlled by automatic means.

While I have herein shown and described several forms of the present invention, it will be understood that various other changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of the present invention.

What I claim is:

1. In a tunnel kiln for firing ceramic ware, said kiln having a preheat zone, a firing zone and a cooling zone in that order, said cooling zone having a rapid cooling sub-zone adjacent said fiiring zone, air circulating means for withdrawing air from said sub-zone and for feeding air thereto, said air circulating means having an inlet and an outlet, means for supplying cold air to said inlet, means for venting said outlet, and automatic temperature responsive means for governing said air supply means and said venting means for varying the two while maintaining the airflow therethrough at a substantially constant ratio.

2. Apparatus as defined in claim 1, wherein said automatic temperature responsive means comprises a first damper controlling said cold air supply means, a second damper controlling said venting means, a radiation pyrometer sighted into said sub-zone, a controller having an output dependent on the output voltage of said radiation pyrometer, motor means for moving said first damper, energizing circuit means for said motor means for supplying energy thereto proportional to said controller output, and means responsive to operation of said first motor means for moving said second damper so that the ratio of volume of air passing through said air inlet means and said damper means remains substantially constant.

3. In a tunnel kiln for firing ceramic ware, said kiln having a preheat zone, a firing zone and a cooling zone in that order, said cooling zone having a rapid cooling subzone adjacent said firing zone, air circulating means for withdrawing air from said sub-zone and for feeding air thereto, said air circulating means having an inlet and an outlet, means for supplying cold air, means for mixing said cold air with the air withdrawn from said sub-zone by said air circulating means on the inlet side of said air circulating means, means communicating with said outlet for venting a portion of the air passing therethrough, and automatic temperature responsive means for governing said air supply means and said venting means for varying the two while maintaining the airflow therethrough at a substantially constant ratio.

4. Apparatus as defined in claim 1, further comprising means for introducing cold air adjacent the exit end of said kiln, and a plurality of longitudinally spaced air circulating means disposed between the exit of said kiln and said sub-zone for withdrawing air from said kiln on the 1 1 exit end of each of said means and for discharging a portion at least of said Withdrawn air into said kiln on the sub-zone end of said means.

5. Apparatus as defined'in claim 1, wherein said automatic temperature responsive means is responsive to the surface temperature of ware between the ends of said sub-zone.

6. Apparatus as defined in claim 2, further comprising a refractory body disposed within said kiln chamber in closely spaced relation with said ware, said refractory body substantially filling the field ofsight of said radiation pyrometer.

7. In a tunnel kiln for firing ceramic ware, said kiln having a preheat zone, a firing zone and a cooling zone in that order, means for introducing cool air adjacent the exit end of said cooling zone, a plurality of longitudinally spaced air circulating means disposed between the exit end of said cooling zone and the firing zone for withdrawing air from said kiln on the exit end of each of said means and for discharging a portion at least of'said withdrawn References Cited in the file of this patent UNITED STATES PATENTS Miller Mar, 15, 1960 Hanley Nov. 15, 1960 OTHER REFERENCES Ahrendt, W. R. Servomechanism Practice, N.Y. Mc- Graw Hill, 1954 pp. 14, 32, 33 T1214. A4. 

1. IN A TUNNEL FOR FIRING CERAMIC WARE, SAID KILN HAVING A PREHEAT ZONE, A FIRING ZONE AND A COOLING ZONE IN THAT ORDER, SAID COOLING ZONE HAVING A RAPID COOLING SUB-ZONE ADJACENT SAID FIRING ZONE, AIR CIRCULATING MEANS FOR WITHDRAWING AIR FROM SAID SUB-ZONE AND FOR FEEDING AIR THERETO, SAID AIR CIRCULATING MEANS HAVING AN INLET AND AN OUTLET, MEANS FOR SUPPLYING COLD AIR TO SAID INLET, MEANS FOR VENTING SAID OUTLET, AND AUTOMATIC TEMPERATURE RESPONSIVE MEANS FOR GOVERNING SAID AIR SUPPLY MEANS AND SAID VENTING MEANS FOR VARYING THE TWO WHILE MAINTAINING THE AIRFLOW THERETHROUGH AT A SUBSTANTIALLY CONSTANT RATIO. 